Waveguide with coherent interference mitigation

ABSTRACT

A pupil-replicating waveguide suitable for operation with a coherent light source is disclosed. A waveguide body has opposed surfaces for guiding a beam of image light. An out-coupling element is disposed in an optical path of the beam for out-coupling portions of the beam at a plurality of spaced apart locations along the optical path. Electrodes are coupled to at least a portion of the waveguide body for modulating an optical path length of the optical path of the beam to create time-varying phase delays between the portions of the beam out-coupled by the out-coupling element.

TECHNICAL FIELD

The present disclosure relates to visual displays and display systems,and in particular to wearable displays, components, modules, and relatedmethods.

BACKGROUND

Head-mounted displays (HMDs), near-eye displays (NEDs), and otherwearable display systems can be used to present virtual scenery to auser, or to augment real scenery with dynamic information, data, orvirtual objects. The virtual or augmented scenery can bethree-dimensional (3D) to enhance the experience and to match virtualobjects to real objects observed by the user. Eye position and gazedirection, and/or orientation of the user may be tracked in real time,and the displayed scenery may be dynamically adjusted depending on theuser's head orientation and gaze direction, to provide a betterexperience of immersion into a simulated or augmented environment.

Lightweight and compact near-eye displays reduce strain on the user'shead and neck, and are generally more comfortable to wear. The opticsblock of such displays can be the heaviest part of the entire system.Compact planar optical components, such as waveguides, gratings, Fresnellenses, etc., may be employed to reduce size and weight of an opticsblock. However, compact planar optics may have limitations related toimage quality, exit pupil size and uniformity, pupil swim, field of viewof the generated imagery, visual artifacts, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described in conjunction with thedrawings, in which:

FIG. 1 is a schematic cross-sectional view of a pupil-replicatingwaveguide of the present disclosure coupled to a coherent light source;

FIG. 2 is a side cross-sectional view of the pupil-replicating waveguideof FIG. 1 showing optical path length difference between out-coupledportions of the beam;

FIG. 3 is a side cross-sectional view of a pupil-replicating waveguideincluding a nanovoided electroactive polymer layer;

FIG. 4 is a side cross-sectional view of a pupil-replicating waveguideincluding a liquid crystal layer;

FIG. 5A is a side cross-sectional view of a pupil-replicating waveguideincluding an acoustic actuator for creating a volume acoustic wave inthe pupil-replicating waveguide;

FIG. 5B is a side cross-sectional view of a pupil-replicating waveguideincluding an acoustic actuator for creating a surface acoustic wave inthe pupil-replicating waveguide;

FIG. 6 is a side cross-sectional view of a wearable display of thepresent disclosure;

FIG. 7 is a flow chart of a method for expanding a beam of image light;

FIG. 8A is an isometric view of an eyeglasses form factor near-eyeaugmented reality (AR)/virtual reality (VR) display incorporating apupil-replicating waveguide of the present disclosure;

FIG. 8B is a side cross-sectional view of the AR/VR display of FIG. 8A;and

FIG. 9 is an isometric view of a head-mounted display (HMD)incorporating a pupil-replicating waveguide of the present disclosure.

DETAILED DESCRIPTION

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives and equivalents, as will be appreciatedby those of skill in the art. All statements herein reciting principles,aspects, and embodiments of this disclosure, as well as specificexamples thereof, are intended to encompass both structural andfunctional equivalents thereof. Additionally, it is intended that suchequivalents include both currently known equivalents as well asequivalents developed in the future, i.e., any elements developed thatperform the same function, regardless of structure.

As used herein, the terms “first”, “second”, and so forth are notintended to imply sequential ordering, but rather are intended todistinguish one element from another, unless explicitly stated.Similarly, sequential ordering of method steps does not imply asequential order of their execution, unless explicitly stated. In FIGS.1 to 4, 5A and 5B, and FIG. 6, similar reference numerals denote similarelements.

Pupil-replicating waveguides are often used in wearable displays due totheir compactness and suitability for augmented reality (AR)applications. It may be desirable to use a high coherence light source,such as a laser source, in a wearable display with pupil-replicatingwaveguide(s). High degree of coherence of light source enables highlyefficient beam redirection and delivery. Laser beam scanned displays canhave good power efficiency, low cost, compact size, bright color gamut,and may enable resolution scaling. However, in a pupil-replicatingwaveguide display, each coherently replicated beam has a differentoptical path length, and therefore a different optical phase. When theeye receives and focuses multiple replicated out-of-phase beams,constructive or destructive interference may result. This causes theintensity of the image to spatially vary in an uncontrollable manner. Inaccordance with the present disclosure, time-varying phase delays may beimparted on different coherently replicated beams to smooth or averageout undesired interference effects. The time-varying phase delays may becreated by varying optical path length of the beam propagating along theoptical path in the waveguide. The optical path length may be varied byvarying physical path length, refractive index, or both.

In accordance with the present disclosure, there is provided apupil-replicating waveguide comprising a waveguide body having opposedsurfaces for guiding a beam of image light therebetween. An out-couplingelement is disposed in an optical path of the beam for out-couplingportions of the beam at a plurality of spaced apart locations along theoptical path. Electrodes are coupled to at least a portion of thewaveguide body for modulating an optical path length of the optical pathof the beam to provide time-varying phase delays between the portions ofthe beam out-coupled by the out-coupling element.

The out-coupling element and one of the electrodes may include a sameelectrically conductive diffraction grating. The waveguide body mayinclude a substrate for propagating the beam of image light therein, andan electrically responsive layer disposed between the electrodes andconfigured to modulate the optical path length of the beam uponapplication of an electrical signal to the electrodes.

In some embodiments, the electrical signal comprises voltage, and theelectrically responsive layer comprises an elastic polymer materialdeformable by an electrostatic attraction force between the electrodesupon application of the voltage. The elastic polymer material mayinclude a nanovoided polymer having a thickness of e.g. between 0.1micrometers and 20 micrometers. In some embodiments, the electricalsignal comprises voltage, and the electrically responsive layercomprises a liquid crystal layer.

In some embodiments, the waveguide body comprises an electro-opticsubstrate disposed between the electrodes, and the electro-opticsubstrate has a refractive index responsive to electric field betweenthe electrodes upon application of voltage to the electrodes. Theelectrode-coupled portion of the waveguide body may include apiezoelectric material for modulating a physical length of the opticalpath of the beam of image light.

In some embodiments, the waveguide body includes a substrate forpropagating the beam of image light therein, and an acoustic actuatorcoupled to the substrate and comprising an electrically responsive layerbetween the electrodes. A thickness of the electrically responsive layeris variable by applying an electrical signal to the electrodes. Theacoustic actuator may be coupled at a side of the substrate andconfigured to provide a volume acoustic wave in the substrate.Alternatively, the acoustic actuator may be mechanically coupled to oneof the surfaces and configured to provide a surface acoustic wave inthat surface.

In accordance with the present disclosure, there is provided a wearabledisplay comprising a light source for providing a beam of image lightcarrying a plurality of image frames at a frame rate, apupil-replicating waveguide, and a controller. The pupil-replicatingwaveguide may include a waveguide body having opposed surfaces forguiding the beam of image light therebetween, an out-coupling element inan optical path of the beam for out-coupling portions of the beam at aplurality of spaced apart locations along the optical path, andelectrodes coupled to at least a portion of the waveguide body formodulating an optical path length of the optical path of the beam tocreate time-varying phase delays between the portions of the beamout-coupled by the out-coupling element. The controller may be operablycoupled to the electrodes of the pupil-replicating waveguide andconfigured to apply an electrical signal to the electrodes to modulatethe optical path length.

A rate of modulation of the optical path length may be higher than theframe rate. In embodiments where each image frame comprises a timesequence of frame lines at a line rate higher than the frame rate, therate of modulation of the optical path length may be higher than theline rate. In embodiments where each frame line comprises a timesequence of line pixels at a pixel rate higher than the line rate, therate of modulation of the optical path length may be higher than thepixel rate. A rate of modulation of the optical path length may be madeto randomly vary relative to a rate at which individual pixels of animage frame are updated.

In accordance with the present disclosure, there is further provided amethod for expanding a beam of image light. The method includespropagating the beam along an optical path in a pupil-replicatingwaveguide, out-coupling, using an out-coupling element in an opticalpath of the beam, portions of the beam at a plurality of spaced apartlocations along the optical path, and modulating, by applying anelectrical signal to electrodes coupled to at least a portion of thepupil-replicating waveguide, an optical path length of the optical pathof the beam to create time-varying phase delays between the portions ofthe beam out-coupled by the out-coupling element. In some embodiments,the electrical signal comprises voltage, and the optical path length ismodulated using an electrically responsive layer between the electrodes,by applying the voltage thereto. In some embodiments, the beam of imagelight carries a plurality of image frames at a frame rate, and a rate ofmodulation of the optical path length is randomly varying relative tothe frame rate.

Referring now to FIG. 1, a pupil-replicating waveguide 100 is opticallycoupled to a coherent light source 102, which provides a cone of beamsof image light carrying an image in angular domain to be displayed to aneye 105. Only one beam 104 is shown for brevity; it is to be understoodthat the image light comprises beams at multiple angles within a fieldof view observable by the eye 105. An in-coupler 103, e.g. a diffractiongrating, may be provided to in-couple the beam 104 for propagation in awaveguide body 106 of the pupil-replicating waveguide. The waveguidebody 106 has opposed top 115 and bottom 116 surfaces for guiding thebeam 104 between the surfaces 115,116 by reflection, e.g. total internalreflection (TIR), from the surfaces 115,116. An out-coupling element 110such as a surface-relief diffraction grating, a volume Bragg grating(VBG), a hologram, etc., can be disposed in an optical path of the beam104 for out-coupling portions 111,112,113,114 of the beam 104 at aplurality of spaced apart locations 121,122,123,124 along the opticalpath. Electrodes 107,108 can be coupled to at least a portion of thewaveguide body 106 for modulating optical path length of the opticalpath of the beam 104 to create time-varying phase delays between theportions 111,112,113,114 of the beam 104 out-coupled by the out-couplingelement 110. It is noted that, although the out-coupling element 110 isshown in FIG. 1 on the outside, that is, supported by the bottomelectrode 107, the order, i.e. relative position, of the out-couplingelement 110 and the bottom electrode 107 may be reversed.

The electrodes 107,108 can modify some property of at least a portion ofthe waveguide body 106, e.g. geometrical dimensions, index ofrefraction, etc., via a suitable mechanism such as electro-optic effect,piezo effect, thermo-optic effect, magneto-optic effect, acousto-opticeffect, photoelasticity, etc., to modulate the optical path length ofthe beam 104 to modulate, i.e. vary in time domain, optical pathdifference between the different beam portions 111,112,113,114. When theoptical path length difference is varied, the interference effectsbetween the different beam portions 111,112,113,114 wash out, i.e. areaveraged out, reducing or even completely eliminating undesirablespatial modulation of optical power density of the image to bedisplayed.

To employ an electro-optic effect, such as Pockels or Kerr effect, thewaveguide body 106 may include an electro-optic substrate between theelectrodes 107,108. The electro-optic substrate may have a refractiveindex responsive to electric field between the electrodes 107,108generated by applying a voltage (i.e. electric potential difference) tothe electrodes 107,108. The electro-optic substrate may be made oflithium niobate, for example. To employ piezo effect, the waveguide body106 may be made of a transparent material exhibiting a piezo effect,e.g. a suitable crystal such as quartz, lithium tantalate, lithiumniobate, etc. When the waveguide body 106 is made out of a transparentpiezoelectric material, a physical length of the waveguide body 106changes upon application of the voltage to the electrodes 107,108, whichcauses the optical path length change of the beam 104. In someembodiments, the diffraction grating 110 may be made conductive, andthus serve as one of the electrodes, e.g. the electrode 107. In otherwords, the diffraction grating 110 may combine the functions of theout-coupling element and one of the electrodes.

Referring to FIG. 2, an optical path length difference d between two112,113 of the out-coupled portions of the beam 104 is furtherillustrated. The waveguide body 106 has a refractive index n andthickness t, and the beam 104 propagates in the waveguide body 106 at anangle θ. The optical path length difference d consists of two equalhalves d/2, as shown, each half d/2=nt/sin θ, and the optical phasedifference ϕ accordingly isϕ=4πnt(λ sin θ)  (1)

where λ is the wavelength of the beam 104.

For a given angle θ and wavelength λ, one can change the relative phaseshift by varying thickness t, refractive index n, or both. At areasonable waveguide body 106 thickness, e.g. 1-2 mm, refractive indexof 1.5-2.2, and angles θ, e.g. 40-65 degrees, very small changes inthickness (tens of nanometers) or refractive index n (of the order of10⁻⁵) are required to produce the maximum phase difference amplitude ofπ. When relative phases of the beam portions 122,123 change, differentinterference patterns will result. By changing the phase difference ϕrapidly over time, one can visually average out interference effects.For example, one may sweep phase difference ϕ over the π magnitudebetween adjacent replicated beam portions 111,112,113,114 at a ratecomparable to, or higher than, the frame rate of the light source 102(FIG. 1).

Referring now to FIG. 3, a pupil-replicating waveguide 300 of thepresent disclosure includes a waveguide body 306 having two portions: asubstrate 328 for propagating the beam 104, and a thin electricallyresponsive layer 330 disposed between electrodes 307,308. Anout-coupling diffraction grating 310 is disposed on the opposite side ofthe substrate 328 as the electrically responsive layer 330. Theout-coupling diffraction grating 310 may also be disposed on a same sideof the substrate 328 as the electrically responsive layer 330. Theelectrically responsive layer 330 can be configured to modulate theoptical path length of the beam 104 upon application of an electricalsignal, such as electric current (when the electrically responsive layerresponds to electric current) or voltage, i.e. difference of electricpotentials (when the electrically responsive layer responds to electricfield), to the electrodes 307,308. Herein, the term “electricallyresponsive layer” means a layer having a thickness or an opticalthickness (the thickness multiplied by refractive index) dependent onthe electrical signal. One advantage is that electrically responsivematerials exist which create a large change in optical path length withmoderate voltages, increasing the magnitude of the corresponding opticalresponse of the electrically responsive layer 330.

In some embodiments of the present disclosure, the electricallyresponsive layer 330 includes an electroactive polymer material, thatis, a polymer material that may change its size or shape in the presenceof an electric field, thus changing the optical path length within thematerial. One type of electroactive polymers is dielectric electroactivepolymer, which is an elastic polymer material deformable by anelectrostatic attraction force between the electrodes when the voltageis applied to the electrodes. Other types of suitable electroactivepolymers may be used, including e.g. ferroelectric electroactivepolymers which maintain a permanent electric polarization that can bereversed or switched by an external electric field.

One drawback of a dielectric polymer is a comparatively large actuationvoltage. In accordance with the present disclosure, the actuationvoltage of a dielectric polymer of the electrically responsive layer 330may be reduced by providing a plurality of voids 332 throughout thedielectric polymer material. Only several voids 332 are shown in FIG. 3for brevity. The voids 332 may be approximately 7 nm to 70 nm in sizeand may occupy between 10% and 90% of the polymer volume, or, in anotherembodiment, between 30% and 70% of the polymer volume. The thickness ofthe electrically responsive layer 330 comprising a nanovoided dielectricpolymer may be between 0.1 micrometers and 20 micrometers, for example.The presence of the voids 332 improves electromechanical response of thepolymer layer and considerably reduces the required maximum drivingvoltage. As low voltage as 3-5V may be enough to produce a significantphase difference (e.g., π) between the replicated beam portions in thenanovoided polymer material. The driving voltage range may accordinglybe 0V-3V; 0V-5V; 0V-12V; or in some embodiments, 0V to 30V. In someembodiments, the diffraction grating 310 may be disposed not on anopposite side as shown in FIG. 3 but on a same side of the substrate 328as the electrically responsive layer 330, e.g. resting on the topelectrode 308, for example. In some embodiments, the diffraction grating310 may be made out of a conductive material and therefore may act asone of the electrodes 307,308.

Turning to FIG. 4, a pupil-replicating waveguide 400 of the presentdisclosure includes a waveguide body 406 having two portions, asubstrate 428 for propagating the beam 104, and a liquid crystal layer430 disposed between electrodes 407,408. An out-coupling diffractiongrating 410 is disposed on the opposite side of the substrate 428 fromthe liquid crystal layer 330. The out-coupling diffraction grating 410may also be disposed on a same side of the substrate 428 as the liquidcrystal layer 430. Upon application of voltage, i.e. a difference ofelectric potentials, between the electrodes 307,308, liquid crystalmolecules change their orientation e.g. due to dipole interaction withthe resulting electric field, thereby changing birefringence of theliquid crystal layer 430. In one embodiment, the liquid crystalmolecules are oriented generally in X-direction, by correspondingalignment layers, not shown. The optical beam 104 is linearly polarizedsuch that it generally propagates in the XY plane as shown at 440. Uponapplication of voltage between the electrodes 407,408, the liquidcrystal molecules become predominantly oriented in the direction of theelectric field, that is, in Y-direction. In such example configuration,the changed birefringence will result in a corresponding change of theoptical phase of the optical beam 104, creating optical phase variationsbetween beam portions 112,113. Different types of crystal layermaterials and configurations may be used including without limitationnematic, cholesteric, or ferroelectric liquid crystals. Spacer-filledgasket 442 may be provided to seal the liquid crystal layer 430 anddefine the thickness of the liquid crystal layer 430. In someembodiments, a ferroelectric liquid crystal material may be used due tosub-millisecond switching speed of ferroelectric liquid crystals.

Referring to FIG. 5A, a pupil-replicating waveguide 500A of the presentdisclosure includes a waveguide body 506A having two portions, asubstrate 528 for propagating the beam 104, and a volume-wave acousticactuator 530A mechanically coupled at a side of the substrate 528joining its top 515 and bottom 516 reflective surfaces. A diffractiongrating 510 out-couples the portions 112,113 of the beam 104. In theembodiment shown, the volume-wave acoustic actuator 530A includes anelectrically responsive layer 532A, e.g. a piezoelectric layer, disposedbetween electrodes 507A,508A. In operation, an electrical signal at ahigh frequency, typically in the range of 1 MHz to 100 MHz or higher, isapplied to the electrodes 507A,508A causing the electrically responsivelayer 532A to oscillate, typically at a frequency of a mechanicalresonance of the electrically responsive layer 532A. The oscillatingthickness of the electrically responsive layer 532A creates a volumeacoustic wave 534A propagating in the substrate 528 in a direction 535,i.e. along the X-axis. The volume acoustic wave 534A modulates opticalpath length by changing the substrate 528 index of refraction due to theeffect of photoelasticity, the geometrical shape of the substrate 528,or both. In some embodiments, an acoustic wave terminator 536A can becoupled to an opposite side of the substrate 528 to absorb the volumeacoustic wave 534A and thus prevent a standing acoustic wave formationin the substrate 528. A standing volume acoustic wave may be undesirablein that it may create distortions in the image being carried by the beam104 towards a user's eye.

Turning to FIG. 5B, a pupil-replicating waveguide 500B of the presentdisclosure includes a waveguide body 506B having two portions, thesubstrate 528 for propagating the beam 104, and a surface-wave acousticactuator 530B mechanically coupled at the top reflective surface 515.Alternatively, the surface-wave acoustic actuator 530B may also becoupled at the bottom reflective surface 516. In the embodiment shown,the surface-wave acoustic actuator 530B includes an electricallyresponsive layer 532B, e.g. a piezoelectric layer, disposed betweenelectrodes 507B,508B. In operation, an electrical signal at a highfrequency, typically in the range of 1 MHz to 100 MHz or higher, isapplied to the electrodes 507B,508B causing the electrically responsivelayer 532B to oscillate. The oscillation of the electrically responsivelayer 532A creates a surface acoustic wave 534B propagating in thesubstrate 528 in the direction 535, i.e. along the X-axis. The surfaceacoustic wave 534B modulates optical path length by changing thesubstrate 528 index of refraction due to the effect of photoelasticity,the geometrical shape of the substrate 528, or both. When the index ofrefraction or the geometrical shape or size of the substrate arechanged, the optical path length of the beam 104 changes. In someembodiments, an acoustic wave terminator 536B can be coupled to anopposite side of the substrate 528 at the same surface, i.e. at the topsurface 515 in FIG. 5B, to absorb the surface acoustic wave 534B andthus prevent a standing acoustic wave formation. A standing surfaceacoustic wave may be undesirable in that it may create distortions inthe image being carried by the beam 104 towards a user's eye.

Referring now to FIG. 6, a wearable display 680, e.g. a near-eye display(NED) or a head-mounted display (HMD), includes a light source 602providing a beam 604 of image light carrying a plurality of image framesto be displayed to a user's eye 605 at a fixed or variable frame rate.In the embodiment shown, the light source 602 includes a linear laserdiode array 601 optically coupled to a linear scanner 609. Each imageframe may include a time sequence of frame lines generated by the linearlaser diode array 601, which are shifted by the linear scanner 609 insynchronism with the generated lines of the image to form an image frameline by line, at a line rate N times higher than the frame rate, where Nis the number of lines in a frame being generated. A single laser diodelight source coupled to a two-dimensional scanner for scanning the beam604 in X and Y planes may also be used. For the single scanned laserdiode source, each frame line includes a time sequence of line pixels ata pixel rate higher than the line rate. The line rate is the frequencyof a scanning line, e.g. in X plane, and the frame rate is the frequencyof the scanning in Y plane.

A pupil-replicating waveguide 600 includes a waveguide body 606including a substrate 628 having opposed top 615 and bottom 616 surfacesfor guiding the beam 604 between the surfaces 615,616 by reflection,e.g. TIR, from the surfaces 615,616, and an electrically responsivelayer 630. The beam 604 may be coupled into the waveguide body 606 byusing a grating coupler 603, for example. Using TIR for guiding the beam604 in the waveguide body 606 has an advantage of allowing an externallight at angles less than a TIR critical angle to be transmitted throughthe surfaces 615,616. Any of the above described pupil-replicatingwaveguides, i.e. the pupil-replicating waveguide 100 of FIG. 1, thepupil-replicating waveguide 300 of FIG. 3, the pupil-replicatingwaveguide 400 of FIG. 4, the pupil-replicating waveguide 500A of FIG.5A, or the pupil-replicating waveguide 500B of FIG. 5B may be used. Anout-coupling element 610, such as a surface-relief diffraction grating,a volume Bragg grating (VBG), a hologram, etc., may be disposed in theoptical path of the beam 604 at any one of the surfaces 615,616, orinside a substrate 628, for out-coupling portions 611,612,613,614 of thebeam 604 at a plurality of spaced apart locations 621,622,623,624 alongthe optical path.

Electrodes 607,608 may be coupled to the waveguide 600, or to anelectrically responsive layer 630 of the waveguide 600. The electricallyresponsive layer 630 may include e.g. the nanovoided elastic polymerlayer 330 of FIG. 3 or the liquid crystal layer 430 of FIG. 4. Theelectrodes 607,608 convey an electrical signal, e.g. electrical currentor voltage, to the electrically responsive layer 630 for modulating anoptical path length of the optical path of the beam 604 to createtime-varying phase delays between the portions 611,612,613,614 of thebeam 604 out-coupled by the out-coupling element 610. A controller 650can be operably coupled to the electrodes 607,608 of and configured toapply the electrical signal to the electrodes 607,608 to modulate theoptical path length. The controller 650 may also be coupled to the lightsource 602 for providing image frames to be displayed.

The controller 650 may be configured to have a rate of modulation of theoptical path length rate to be higher than the frame rate, such that anyoptical power density non-uniformities due to interference betweendifferent beam portions 611-614 may be averaged out in a single frame.For embodiments where the light source 602 includes the linear diodearray 601 coupled to a linear scanner 609, the controller 650 may beconfigured to have the rate of modulation of the optical path lengthrate to be higher than the line rate, such that any optical powerdensity non-uniformities due to interference between different beamportions 611-614 may be averaged out in each image line. For embodimentswhere a single diode laser light source is scanned in two dimensions,e.g. in X and Y planes, the controller 650 may be configured to have therate of modulation of the optical path length higher than the pixelrate. It may also be preferable to configure the controller 650 suchthat the rate of modulation of the optical path length is randomlyvarying relative to the frame rate, or relative to a rate at whichindividual pixels of an image frame are updated. In other words, thephase delays of the beam portions 611-614 may be made random relative tothe time intervals when frames, lines, and/or individual pixels of theimage are being updated. This may prevent the interference-caused imagenon-uniformity patterns from staying steady or drifting across imageframes.

Referring now to FIG. 7, a method 700 for expanding a beam of imagelight includes propagating (702) the beam along an optical path in apupil-replicating waveguide. Portions of the beam are out-coupled (704)using an out-coupling element in an optical path of the beam at aplurality of spaced apart locations along the optical path. Optical pathlength of the optical path of the beam is then modulated (706) byapplying an electrical signal to electrodes coupled to at least aportion of the waveguide, to create time-varying phase delays betweenthe portions of the beam out-coupled by the out-coupling element. Themodulated optical path length causes interference patterns of theout-coupled beam portions to shift, effectively washing them out, oraveraging them. This enables one to use a coherent light source, whichmay be well-collimated and may be easier to work with.

In some embodiments of the method 700, the rate of modulation of theoptical path length is randomly varying relative to the frame rate ofthe images displayed, to prevent any noticeable steady or driftinginterference patterns from appearing. The optical path length modulationmay be carried out using any of the devices/technologies describedabove, e.g. using an electrically responsive layer between theelectrodes, such as a nanovoided polymer, a liquid crystal layer, apiezo element, etc.

Referring to FIGS. 8A and 8B, a near-eye AR/VR display 800 is anembodiment of the wearable display 680 of FIG. 6, and may include thepupil-replicating waveguide 100 of FIG. 1, the pupil-replicatingwaveguide 300 of FIG. 3, the pupil-replicating waveguide 400 of FIG. 4,the pupil-replicating waveguide 500A of FIG. 5A, and/or thepupil-replicating waveguide 500B of FIG. 5B. A body or frame 802 of thenear-eye AR/VR display 800 has a form factor of a pair of eyeglasses, asshown. A display 804 includes a display assembly 806 (FIG. 8B), whichprovides image light 808 to an eyebox 810, i.e. a geometrical area wherea good-quality image may be presented to a user's eye 812. The displayassembly 806 may include a separate coherent-replication VR/AR displaymodule for each eye, or one coherent-replication VR/AR display modulefor both eyes. For the latter case, an optical switching device may becoupled to a single electronic display for directing images to the leftand right eyes of the user in a time-sequential manner, one frame forleft eye and one frame for right eye. The images are presented fastenough, i.e. with a fast enough frame rate, that the individual eyes donot notice the flicker and perceive smooth, steady images of surroundingvirtual or augmented scenery.

An electronic display of the display assembly 806 may include, forexample and without limitation, a liquid crystal display (LCD), a LiquidCrystal on Silicon (LCoS) display, a scanned laser beam display, ascanned laser beam array, a phased array display, a holographic display,or a combination thereof. The near-eye AR/VR display 800 may alsoinclude an eye-tracking system 814 for determining, in real time, thegaze direction and/or the vergence angle of the user's eyes 812. Thedetermined gaze direction and vergence angle may also be used forreal-time compensation of visual artifacts dependent on the angle ofview and eye position. Furthermore, the determined vergence and gazeangles may be used for interaction with the user, highlighting objects,bringing objects to the foreground, dynamically creating additionalobjects or pointers, etc. Furthermore, the near-eye coherent AR/VRdisplay 800 may include an audio system, such as small speakers orheadphones.

Turning now to FIG. 9, an HMD 900 is an example of an AR/VR wearabledisplay system enclosing user's eyes for a greater degree of immersioninto the AR/VR environment. The HMD 900 may be a part of an AR/VR systemincluding a user position and orientation tracking system, an externalcamera, a gesture recognition system, control means for providing userinput and controls to the system, and a central console for storingsoftware programs and other data for interacting with the user forinteracting with the AR/VR environment. The functional purpose of theHMD 900 is to augment views of a physical, real-world environment withcomputer-generated imagery, and/or to generate entirely virtual 3Dimagery. The HMD 900 may include a front body 902 and a band 904. Thefront body 902 is configured for placement in front of eyes of the userin a reliable and comfortable manner, and the band 904 may be stretchedto secure the front body 902 on the user's head. A display system 980may include any of the waveguide assemblies described herein. Thedisplay system 980 may be disposed in the front body 902 for presentingAR/VR images to the user. Sides 906 of the front body 902 may be opaqueor transparent.

In some embodiments, the front body 902 includes locators 908, aninertial measurement unit (IMU) 910 for tracking acceleration of the HMD900 in real time, and position sensors 912 for tracking position of theHMD 900 in real time. The locators 908 may be traced by an externalimaging device of a virtual reality system, such that the virtualreality system can track the location and orientation of the HMD 900 inreal time. Information generated by the IMU and the position sensors 912may be compared with the position and orientation obtained by trackingthe locators 908, for improved tracking of position and orientation ofthe HMD 900. Accurate position and orientation is important forpresenting appropriate virtual scenery to the user as the latter movesand turns in 3D space.

The HMD 900 may further include an eye tracking system 914, whichdetermines orientation and position of user's eyes in real time. Theobtained position and orientation of the eyes allows the HMD 900 todetermine the gaze direction of the user and to adjust the imagegenerated by the display system 980 accordingly. In one embodiment, thevergence, that is, the convergence angle of the user's eyes gaze, isdetermined. The determined gaze direction and vergence angle may be usedfor real-time compensation of visual artifacts dependent on the angle ofview and eye position. Furthermore, the determined vergence and gazeangles may be used for interaction with the user, highlighting objects,bringing objects to the foreground, creating additional objects orpointers, etc. An audio system may also be provided including e.g. a setof small speakers built into the front body 902.

The hardware used to implement the various illustrative logics, logicalblocks, modules, and circuits described in connection with the aspectsdisclosed herein may be implemented or performed with a general purposeprocessor, a digital signal processor (DSP), an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA) orother programmable logic device, discrete gate or transistor logic,discrete hardware components, or any combination thereof designed toperform the functions described herein. A general-purpose processor maybe a microprocessor, but, in the alternative, the processor may be anyconventional processor, controller, microcontroller, or state machine. Aprocessor may also be implemented as a combination of computing devices,e.g., a combination of a DSP and a microprocessor, a plurality ofmicroprocessors, one or more microprocessors in conjunction with a DSPcore, or any other such configuration. Alternatively, some steps ormethods may be performed by circuitry that is specific to a givenfunction.

The present disclosure is not to be limited in scope by the specificembodiments described herein. Indeed, other various embodiments andmodifications, in addition to those described herein, will be apparentto those of ordinary skill in the art from the foregoing description andaccompanying drawings. Thus, such other embodiments and modificationsare intended to fall within the scope of the present disclosure.Further, although the present disclosure has been described herein inthe context of a particular implementation in a particular environmentfor a particular purpose, those of ordinary skill in the art willrecognize that its usefulness is not limited thereto and that thepresent disclosure may be beneficially implemented in any number ofenvironments for any number of purposes. Accordingly, the claims setforth below should be construed in view of the full breadth and spiritof the present disclosure as described herein.

What is claimed is:
 1. A pupil-replicating waveguide comprising: awaveguide body having opposed surfaces for guiding a beam of image lighttherebetween; an out-coupling element in an optical path of the beam forout-coupling a plurality of portions of the beam at a plurality ofspaced apart locations along the optical path; and electrodes coupled toat least a portion of the waveguide body for modulating an optical pathlength of the optical path of the beam to provide time-varying phasedelays between different portions of the plurality of portions of thebeam out-coupled by the out-coupling element.
 2. The pupil-replicatingwaveguide of claim 1, wherein the out-coupling element and one of theelectrodes comprise a same electrically conductive diffraction grating.3. The pupil-replicating waveguide of claim 1, wherein the waveguidebody comprises: a substrate for propagating the beam of image lighttherein; and an electrically responsive layer disposed between theelectrodes and configured to modulate the optical path length of thebeam upon application of an electrical signal to the electrodes.
 4. Thepupil-replicating waveguide of claim 3, wherein the electrical signalcomprises voltage, and wherein the electrically responsive layercomprises an elastic polymer material deformable by an electrostaticattraction force between the electrodes upon application of the voltage.5. The pupil-replicating waveguide of claim 4, wherein the elasticpolymer material comprises a nanovoided polymer.
 6. Thepupil-replicating waveguide of claim 5, wherein the nanovoided polymerhas a thickness of between 0.1 micrometers and 20 micrometers.
 7. Thepupil-replicating waveguide of claim 3, wherein the electrical signalcomprises voltage, and wherein the electrically responsive layercomprises a liquid crystal layer.
 8. The pupil-replicating waveguide ofclaim 1, wherein the waveguide body comprises an electro-optic substratedisposed between the electrodes, and wherein the electro-optic substratehas a refractive index responsive to electric field between theelectrodes upon application of voltage thereto.
 9. The pupil-replicatingwaveguide of claim 8, wherein the at least a portion of the waveguidebody comprises a piezoelectric material for modulating a physical lengthof the optical path of the beam of image light.
 10. Thepupil-replicating waveguide of claim 1, wherein the waveguide bodycomprises: a substrate for propagating the beam of image light therein;and an acoustic actuator coupled to the substrate and comprising anelectrically responsive layer between the electrodes, wherein athickness of the electrically responsive layer is variable by applyingan electrical signal to the electrodes.
 11. The pupil-replicatingwaveguide of claim 10, wherein the acoustic actuator is coupled at aside of the substrate and configured to provide a volume acoustic wavein the substrate.
 12. The pupil-replicating waveguide of claim 10,wherein the acoustic actuator is mechanically coupled to one of thesurfaces and configured to provide a surface acoustic wave in thatsurface.
 13. A wearable display comprising: a light source for providinga beam of image light carrying a plurality of image frames at a framerate; a pupil-replicating waveguide comprising: a waveguide body havingopposed surfaces for guiding the beam of image light therebetween; anout-coupling element in an optical path of the beam for out-coupling aplurality of portions of the beam at a plurality of spaced apartlocations along the optical path; and electrodes coupled to at least aportion of the waveguide body for modulating an optical path length ofthe optical path of the beam to create time-varying phase delays betweendifferent portions of the plurality of portions of the beam out-coupledby the out-coupling element; and a controller operably coupled to theelectrodes of the pupil-replicating waveguide and configured to apply anelectrical signal to the electrodes to modulate the optical path length.14. The wearable display of claim 13, wherein a rate of modulation ofthe optical path length is higher than the frame rate.
 15. The wearabledisplay of claim 14, wherein each image frame comprises a time sequenceof frame lines at a line rate higher than the frame rate, and whereinthe rate of modulation of the optical path length is higher than theline rate.
 16. The wearable display of claim 14, wherein each frame linecomprises a time sequence of line pixels at a pixel rate higher than theline rate, and wherein the rate of modulation of the optical path lengthis higher than the pixel rate.
 17. The wearable display of claim 14,wherein a rate of modulation of the optical path length is randomlyvarying relative to a rate at which individual pixels of an image frameare updated.
 18. A method for expanding a beam of image light, themethod comprising: propagating the beam along an optical path in apupil-replicating waveguide; out-coupling, using an out-coupling elementin an optical path of the beam, a plurality of portions of the beam at aplurality of spaced apart locations along the optical path; andmodulating, by applying an electrical signal to electrodes coupled to atleast a portion of the pupil-replicating waveguide, an optical pathlength of the optical path of the beam to create time-varying phasedelays between different portions of the plurality of portions of thebeam out-coupled by the out-coupling element.
 19. The method of claim18, wherein the electrical signal comprises voltage, and wherein theoptical path length is modulated using an electrically responsive layerbetween the electrodes, by applying the voltage thereto.
 20. The methodof claim 18, wherein the beam of image light carries a plurality ofimage frames at a frame rate, and wherein a rate of modulation of theoptical path length is randomly varying relative to the frame rate.